Chromatic dispersion, or group-velocity dispersion, in optical communication systems is caused by a variation in the group velocity in a fiber with changes in optical frequency. This dispersion can cause pulse spreading in a lightwave signal, resulting in impaired system performance. Pulse spreading is especially troublesome in WDM transmission systems where two or more optical data carrying channels are combined onto a common path for transmission to a remote receiver.
When such a pulse, which may contain a spectrum of wavelengths, traverses the fiber, different wavelength components travel at different velocities. Thus, the pulse broadens as it travels down the fiber and by the time it reaches the receiver, it may have spread over several bit periods. This spreading may cause transmission errors. Furthermore, the wavelength dependence of chromatic dispersion is significant in long WDM systems because different wavelengths may need different dispersion compensation.
An NRZ modulation format is used to send binary information characterized by a light pulse that is rectangular and occupies the entire bit period. In contrast, in a Return-to-Zero (RZ) format, the light pulse occupies about half the bit period. The term NRZ describes the waveform's constant value characteristic when consecutive binary “ones” are sent. For example, if three binary “ones” are transmitted in a row, then the resulting waveform is a rectangle extending three entire bit periods, without returning back to zero.
The choice of the modulation format in WDM systems is a compromise between spectral efficiency and resistance against nonlinear propagation effects. The NRZ transmission format is particularly useful for transmitting large amounts of data over optically amplified fiber paths. As compared to Chirped-Return-to Zero (CRZ) (chirped RZ) or RZ, the NRZ modulation format is spectrally more efficient but is less resistant to nonlinearities. Consequently, known NRZ-based high-capacity WDM systems commonly operate at a low power per channel and usually over shorter distances to avoid these severe nonlinearities.
As NRZ-based systems provide a high degree of spectral efficiency, it would be desirable to decrease the nonlinear transmission penalties associated with NRZ format in long-haul WDM systems. This would allow systems to run at an increased power per channel and/or at longer distances.
Several known methods address the need for an NRZ-based WDM system with reduced nonlinearities. One group is known as pre-compensation (pre-launch compensation). Another group is known as post-compensation (compensation at the receiver terminal). Another known method of dispersion compensation is performed along the transmission line and known as dispersion mapping.
Dispersion mapping uses dispersion-shifted optical fiber as the preferred transmission medium. Through this technique, the zero dispersion wavelengths of the transmission fiber are offset from the operating wavelengths of the transmitter. The technique employs a series of amplifier sections having dispersion shifted fiber spans with either positive or negative dispersion. The dispersion accumulates over multiple fiber spans of approximately 500 to 1000 km. The fiber spans of either a positive or negative sign are followed by a dispersion-compensating fiber having dispersion of the opposite sign. This subsequent section of fiber is sufficient to reduce the average dispersion over the total length of the transmission system substantially to a zero.
The dispersion mapping technique is limited because the amount of dispersion that occurs in a typical optical fiber depends on the operating wavelength that is employed. This shortcoming may be overcome to a limited degree by using individual channel dispersion compensation at the receiver (post-compensation). However, because these systems are subject to nonlinear penalties, the ability to correct the non-zero dispersion at the receiver terminal is also limited.
Another approach is described in U.S. Pat. No. 6,137,604. In the '604 patent, a method and apparatus is provided wherein the usable optical bandwidth of the transmission system is divided into sub-bands that individually undergo dispersion compensation before being re-combined. In this way, more WDM data channels reside near a wavelength corresponding to the average zero dispersion wavelength. However, for NRZ-based WDM systems, nonlinearities are still somewhat troublesome and not completely reduced, especially near the edge channels of a given band.
A further approach compensates for the line dispersion in the terminal dispersion compensation units (DCUs). This results in a residual dispersion very close to zero. The residual dispersion is the sum of the total amount of dispersion compensation and the accumulated dispersion in the transmission line, including the dispersion sign. Again, nonlinearities still exist, which effect the amount of power per channel that can be used as well as the total length of the system.
The above methods have certain disadvantages. Although those techniques are sufficient in reducing nonlinearities, there is still a need for improvement, especially where a given NRZ-based WDM system has strong nonlinearities at edge channels of a given band. Here, the channels accumulate a large amount of dispersion along the transmission line. It would be desirable to reduce the amount of nonlinearities while having an increased optical power per channel and increased system distances.